Does anyone know what is the difference when people make reference to Sub-Filter Scales or Sub-grid Scales modelling. Are they the same, or do they have different meanings? I know what is classicaly called SGS, but I am not sure about SFS... However I would think it would be more apropriated to call SFS what is used called SGS for obvious reasons, but I understood there are some traditional nomenclature convention about it. I just dont know what people agreed to call SFS. Please can anyone provide a insight?

SGS - these are the scales smaller than the cell size. SFS - in dynamic modeling, test filters are used and hence scales smaller than the filter size, but greater than the grid size would be the SFS

My answer slightly differs from Agg's one.

Whatever the subgrid model are (i.e. requiring or not a test filter), if you perform an explicit filtering (as advocated by some LES purists), the flowfield is divided into 3 parts:

1) the resolved field

2) the subfilter field, which is subtracted from the resolveld field by the explicit filtering but whose characteristic wavelengths are high enough to be (non accurately) discretized on the mesh

3) the subgrig field, whose wavelengths are lower than 2*delta_x (implicit filtering by the grid: most LES rely on this filtering only).

Hope this help